Paper comprises, in addition to cellulose fiber, a variety of additives such as fillers, strength agents and the like. Wet strength agents are an additive which gives paper having increased strength in the wet state. Examples of widely used wet strength agents include urea resins, melamine resins, dialdehyde starch, polyethyleneimine, epoxylated polyamide etc.
Recently, several attempts have been made to use polyurethanes and urethane prepolymers as wet strength agent. JP-A-06173196 discloses use of a urethane prepolymer containing tertiary amine or quaternary ammonium moieties without blocking the free isocyanate groups. The prepolymer is either added into the stock or applied to webs formed therefrom in the form of an aqueous dispersion or emulsion. GB 2068034A discloses use of a urethane prepolymer amine salt as wet strength agent. The prepolymer amine salt is produced by blocking a urethane prepolymer having free isocyanate groups with a ketoxime, and reacting the blocked urethane prepolymer with a polyfunctional amine to give amine containing-blocked urethane prepolymer followed by dissolving the reaction product in water in the form of an acid addition salt. This amine salt is either added to the stock or applied to a web formed therefrom. JP-A-05051896 discloses a wet strength agent of the type to be applied to a web or paper. The agent comprises a water-soluble urethane prepolymer blocked with sodium hydrogen sulfite or a water soluble urethane prepolymer blocked with ketoxime or other blocking agent which is solubilized by introducing a hydrophilic moiety using the reaction with dimethylolpropionic acid.
Urethane prepolymers containing free isocyanate groups are generally unstable in the presence of water. Blocked urethane prepolymers, on other hand, require heating for unblocking. For blocked urethane prepolymers to be added in the stock, it is imperative to be unblocked under conditions to be encountered in the dryer part of paper making machines, typically at a temperature of up to 130° C. for several minutes. For a wet strength agent to be added in the stock, it is important to be adsorbed to and retained by cellulosic fiber in the stock.
Accordingly, a need exists for a wet strength composition of the type to be added to the paper making stock which contains a water-soluble blocked urethane prepolymer capable of unblocking under conditions to be normally encountered in the dryer part of paper making machines and which may be adsorbed to and retained by cellulosic fiber in the stock to the greatest extent as possible.